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ISO 10646/Unicode and MIME

1993-12-15 14:37:53
Enclosed please find the document discussing general 10646/Unicode issues
in MIME, in both ASCII and PostScript form.


Encoding of ISO/IEC 10646-1/Unicode in MIME

Mark Davis and David Goldsmith
mark_davis(_at_)taligent(_dot_)com
david_goldsmith(_at_)taligent(_dot_)com

Status of this Memo

This document is a preliminary proposal, intended to be eventually
submitted as an Internet standard. This draft is for discussion
purposes only.

Abstract

ISO/IEC 10646-1:1993(E) and the Unicode Standard, version 1.1,
jointly define a 16 bit character set (hereafter referred to as BMP,
the Basic Multilingual Plane of 10646) which encompasses most of the
world's writing systems. However, Internet mail (STD 11, RFC 822)
currently supports only 7-bit US ASCII as a character set. MIME (RFC
1521 and RFC 1522) extends Internet mail to support different media
types and character sets, and thus could support BMP in mail
messages. However, MIME neither defines BMP as a permitted character
set nor specifies how it would be encoded.

This document is a proposed addition to RFC 1521 and RFC 1522
specifying the encoding of ISO/IEC 10646-1/Unicode within MIME. It
references a companion document, "UTF-7: A Mail Safe Transformation
Format of ISO/IEC 10646-1/Unicode".

Motivation

Since BMP is starting to see widespread commercial adoption, users
will want a way to transmit information in this character set in mail
messages and other Internet media. Since MIME was expressly designed
to allow such extensions and is on the standards track for the
Internet, it is the most appropriate means for encoding BMP. RFC 1521
and RFC 1522 currently do not define BMP as an allowed character set.

In addition to allowing use of BMP within MIME bodies, another goal
is to specify a way of using BMP that allows text which consists
largely, but not entirely, of US-ASCII characters to be represented
in a way that can be read by mail clients who do not understand BMP.
This is in keeping with the philosophy of MIME.

Overview

Two ways of using BMP are specified. The first is a straightforward
use of BMP as specified in the ISO/IEC 10646-1:1993(E) document. The
second is based on the transformation format UTF-7.

The first encoding is intended for situations where sender and
recipient do not want to do a lot of processing, or when the text
does not consist primarily of characters from the US-ASCII character
set.

The second encoding is intended for situations where the text
consists primarily of US-ASCII, with occasional characters from other
parts of BMP. This encoding allows the US-ASCII portion to be read by
all recipients without having to support BMP.

Finally, in keeping with the principles set forth in RFC 1521, text
which can be encoded using the US-ASCII or ISO-8859-x character sets
should be so encoded where possible, for maximum interoperability.
[Use of UTF-7 keeps to the spirit if not the letter of this
principle, since it reduces to (mostly) US-ASCII in the limiting
case.]

Definitions

The definition of character set BMP:

The 16 bit character set BMP is defined by the international standard
ISO/IEC 10646-1:1993(E); Coded Representation Form=UCS-2; Subset=300;
Implementation Level=3. This character set is identical with the
character repertoire and coding of The Unicode Standard, Version 1.1.

Note. Unicode 1.1 further specifies the use and interaction of these
character codes beyond the ISO standard. However, any valid BMP
sequence is a valid Unicode sequence; Unicode supplies
interpretations of sequences on which the ISO standard is silent as
to interpretation.

This character set is encoded as sequences of octets, two per 16-bit
character, with the most significant octet first. Text with an odd
number of octets is ill-formed.

Rationale. ISO/IEC 10646-1:1993(E) specifies that when characters in
the UCS-2 form are serialized as octets, that the most significant
octet appear first. This is also in keeping with common network
practice of choosing a canonical format for transmission.

Character set T is the proposed standard transformation format of
BMP, as defined in the document  "UTF-7: A Mail Safe Transformation
Format of ISO/IEC 10646-1/Unicode".

Encoding Character Set BMP Within MIME

Character set BMP uses 16 bit characters, and therefore may only be
used with the Binary or Base64 content transfer encodings of MIME. In
header fields, it may only be used with the B content transfer
encoding. The MIME character set identifier is ISO-10646-UNICODE.

Rationale. There is no other succinct identification for this
character set that might not be confused with other variants of
ISO/IEC 10646-1. Other choices such as ISO-10646-BMP or
ISO-10646-UCS-2 are ambiguous.
ISO-10646-1-1993-E-UCS-2-SUBSET-300-LEVEL-3 is fairly unambiguous,
but we presumed that people would prefer brevity.

Example. Here is a text portion of a MIME message containing the word
"nihongo" (hexadecimal 65E5,672C,8A9E) written in Han characters.

Content-Type: text/plain; charset=ISO-10646-UNICODE
Content-Transfer-Encoding: base64

ZeVnLIqe

Example. Here is a text portion of a MIME message containing the BMP
sequence "A<NOT IDENTICAL TO><ALPHA>." (hexadecimal
0041,2262,0391,002E)

Content-Type: text/plain; charset=ISO-10646-UNICODE
Content-Transfer-Encoding: base64

AEEiYgORAC4=


Encoding Character Set T Within MIME

Character set T is safe for mail transmission and therefore may be
used with any content transfer encoding in MIME. Specifically, the 7
bit encoding for bodies and the Q encoding for headers are both
acceptable. The MIME character set identifier is ISO-10646-UTF7.
Example. Here is a text portion of a MIME message containing the BMP
sequence "Hi Mom <WHITE SMILING FACE>!" (hexadecimal 0048, 0069,
0020, 004D, 006F, 004D, 0020, 263A, 0021).

Content-Type: text/plain; charset=ISO-10646-UTF7

Hi Mom +Jjo!

Example. Here is a text portion of a MIME message containing the BMP
sequence representing the Han characters for the Japanese word
"nihongo" (hexadecimal 65E5,672C,8A9E).

Content-Type: text/plain; charset=ISO-10646-UTF7

+ZeVnLIqe-

Example. Here is a text portion of a MIME message containing the BMP
sequence "A<NOT IDENTICAL TO><ALPHA>." (hexadecimal
0041,2262,0391,002E).

Content-Type: text/plain; charset=ISO-10646-UTF7

A+ImIDkQ.

Example. Here is a text portion of a MIME message containing the BMP
sequence "Item 3 is <POUND SIGN>1."  (hexadecimal 0049, 0074, 0065,
006D, 0020, 0033, 0020, 0069, 0073, 0020, 00A3, 0031, 002E).

Content-Type: text/plain; charset=ISO-10646-UTF7

Item 3 is +AKM-1.

Discussion

In this section we will motivate the introduction of UTF-7 as opposed
to the alternative of using the existing encodings of BMP (e.g.
UTF-FSS) with MIME's content transfer encodings. Before discussing
this, it will be useful to list some assumptions about character
frequency within typical natural language text strings that we use to
estimate typical storage requirements:

1. Most Western European languages use roughly 7/8 of their letters
from US-ASCII and 1/8 from Latin 1 (ISO-8859-1).
2. Most non-European alphabet-based languages (e.g., Greek) use about
1/6 of their letters from ASCII (since white space is in the 7-bit
area) and the rest from their alphabets.
3. East Asian ideographic-based languages (including Japanese) use
essentially all of their characters from the Han or CJK syllabary
area.
4. The = character does not occur frequently enough to affect the
results.

Notice that current 8 bit standards, such as ISO-8859-x, require use
of a content transfer encoding. For comparison with the subsequent
discussion, the costs break down as follows (note that many of these
figures are approximate since they depend on the exact composition of
the text):

8859-x in Base64
Text type           Average octets/character
All                 1.33

8859-x in Quoted Printable
Text type           Average octets/character
US-ASCII            1
Western European    1.25
Other               2.67

Note also that BMP encoded in Base64 takes a constant 2.66 octets per
character. For purposes of comparison, we will look at UTF-FSS in
Base64 and Quoted Printable, and UTF-7. UTF-1 gives results
substantially similar to UTF-FSS. Also note that fixed overhead for
long strings is relative to 1/n, where n is the encoded string length
in octets.

UTF-FSS in Base64 
Text type           Average octets/character
US-ASCII            1.33
Western European    1.5
Some Alphabetics    2.44
All others          4

UTF-FSS in Quoted Printable
Text type           Average octets/character
US-ASCII            1
Western European    1.63
Some Alphabetics    5.17
All others          7-9

UTF-7
Text type           Average octets/character
Most US-ASCII       1
Western European    1.5
All others          2.67+2/n

We feel that the UTF-FSS in Quoted Printable option is not viable due
to the very large expansion of all text except Western European. This
is only viable in texts consisting of large expanses of US-ASCII or
Latin characters with occasional other characters interspersed. We
would prefer to introduce one encoding that works reasonably well for
all users.

We also feel that UTF-FSS in Base64 has high expansion for
non-Western-European users, and is less desirable because it cannot
be read directly, even when the content is largely US-ASCII. The base
encoding of UTF-7 gives competitive results and is readable for ASCII
text.

UTF-7 gives results competitive with ISO-8859-x, with access to all
of the BMP character set. We believe this justifies the introduction
of a new transformation format of BMP.

As an alternative to use of UTF-7, it is possible to intermix BMP
characters with other character sets using an existing MIME
mechanism, the multipart/mixed content type (thanks to Nathaniel
Borenstein for pointing this out). For instance (repeating an earlier
example):

Content-type: multipart/mixed; boundary=foo 

--foo 
Content-type: text/plain; charset=us-ascii 

Hi Mom 
--foo 
Content-type: text/plain; charset=ISO-10646-UNICODE 
Content-transfer-encoding: base64 

Jjo=
--foo 
Content-type: text/plain; charset=us-ascii 

!
--foo-- 

Theoretically, this removes the need for UTF-7. However, we feel that
as use of the BMP character set becomes more widespread, intermittent
use of specialized BMP characters (such as dingbats and mathematical
symbols) will occur, and that text will also typically include small
snippets from other script systems, such as Cyrillic, Greek, or East
Asian languages (anything in the Roman script system is already
handled adequately by existing MIME character sets). Although the
multipart technique works well for large chunks of text in
alternating character sets, we feel it does not adequately support
the kinds of uses just discussed, and so we still believe the
introduction of UTF-7 is justified.

Summary

To be added later.

References

To be added later.
ISO/IEC 10646-1:1993(E); Unicode v1, v2, 1.1 TR; RFC 822, 1521, 1522;
UTF-7; UTF-2 (X/Open)

Acknowledgements

Many thanks to the following people for their helpful comments and
suggestions:
Nathaniel Borenstein, Lee Collins, John Jenkins.
[more later]


Attachment: MIME-Unicode.ps
Description: PostScript document

----------------------------
David Goldsmith
david_goldsmith(_at_)taligent(_dot_)com
Taligent, Inc.
10201 N. DeAnza Blvd.
Cupertino, CA  95014-2233
(408) 777-5225


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